1. Field of the Invention
The present invention generally relates to a mobile communication system. More particularly, the present invention relates to a method for efficiently signaling resource assignment information in a Frequency Division Multiple Access (FDMA) system.
2. Description of the Related Art
Owing to the rapid development of communications technology, mobile communication systems have reached a developmental stage that provides a high-speed data service for sending a large amount of digital data such as e-mails, still images, and moving pictures at high rate to User Equipment (UE) or Mobile Stations (MSs), beyond the traditional voice service.
In this context, Orthogonal Frequency Division Multiplexing (OFDM) has recently been studied and is being actively exploited for the mobile communication systems. OFDM is a special case of Multi-Carrier Modulation (MCM) in which an input serial symbol sequence is parallelized and modulated to mutually orthogonal subcarriers or subcarrier channels, prior to transmission.
FIG. 1 provides an example of time-frequency resource allocation in an OFDM mobile communication system to which the present invention is applied. Referring to FIG. 1, one modulation symbol (e.g. Quadrature Phase Shift Keying (QPSK) or 16-ary Quadrature Amplitude Modulation (16QAM) symbol) is sent, generally on a single subcarrier. The subcarrier is a resource unit in the high-speed data service.
In FIG. 1, the horizontal axis represents time and the vertical axis represents frequency. Reference numeral 101 denotes a subcarrier and reference numeral 102 denotes an OFDM symbol. As illustrated in FIG. 1, one OFDM symbol is composed of a plurality of subcarriers.
The OFDM system sends a plurality of OFDM symbols as a transmission unit called Transmission Time Interval (TTI), as denoted by reference numeral 103. In FIG. 1, a single rectangle defines a time-frequency bin and one TTI has a plurality of time-frequency bins.
Typically, one TTI includes a plurality of physical channels. The physical channels are channels that carry different types of information in the mobile communication system, including paging, packet data, packet data control and uplink scheduling channels.
For example, part of a TTI, that is, some of time-frequency bins, are used for a paging channel, a common control channel carrying system information, a packet data channel carrying user data and a packet data control channel carrying control information required for demodulation of the packet data channel, as illustrated in FIG. 1. While not specified herein, other physical channels may exist to serve other purposes and thus part of the time-frequency bins may be occupied by these other physical channels.
As described above, the OFDM mobile communication system uses two-dimensional resources, i.e. time-frequency resources which are divided into smaller areas for allocation to a plurality of UEs. Because the UEs require different amounts of resources, resources (time-frequency bins) allocated to them should be efficiently agreed on or determined between a transmitter and receivers.
For instance, given 5000 time-frequency bins in one TTI, if the transmitter allocates time-frequency bins 1 to 100 to a first receiver and time-frequency bins 101 to 600 to a second receiver, it should be able to effectively notify the receivers of the resource allocation.
Yet, it is very inefficient to indicate the allocated subcarriers one by one, for example, by specifying the indexes of the individual subcarriers on the frequency axis and the indexes of OFDM symbols with the subcarriers on the time axis because this resource allocation notification requires too much control information. Hence, a resource allocation scheme is under discussion, that groups time-frequency resources into resource blocks and allocates resources to UEs on a resource block basis.
FIG. 2 illustrates allocation of successive time-frequency resources as resource blocks and FIG. 3 illustrates allocation of scattered time-frequency resources as resource blocks.
A resource block with subcarriers successive in time and contiguous in frequency is called a localized resource block, as illustrated in FIG. 2. A resource block with subcarriers successive in time but distributed in frequency is called a distributed resource block, as illustrated in FIG. 3. In the distributed resource allocation of FIG. 3, spaced subcarriers are allocated in the same time area. As a result, the block-based resource allocation illustrated in FIGS. 2 and 3 reduces uplink signaling overhead.
FIG. 4 illustrates transmission of resource assignment information to UEs to which a Base Station (BS) allocates a plurality of resource blocks.
To notify a particular UE by allocation of a resource block, the BS basically uses the Identifier (ID) of the UE and resource block information. The UE ID is about 10 bits long and the resource block information occupies about 5 to 10 bits to deliver more accurate information.
The conventional resource assignment information signaling method increases signaling overhead because UE IDs and resource block information are indicated one by one, as explained above.
To avert this problem, only the UE IDs can be indicated in relation to resource blocks allocated to the UEs with the UE Ids, without explicitly signaling the resource block information.
Referring to FIG. 4, reference numeral 314 denotes resources available to the BS and reference numerals 307 to 313 denote logical resource blocks allocated to UEs. The resource allocation can be performed in a localized manner or a distributed manner, as illustrated in FIG. 2 or FIG. 3. Reference numeral 304 denotes the UE IDs of the UEs and reference numeral 306 denotes short IDs that identify the UEs to which resource blocks are allocated.
The BS first sends the UE IDs to the UEs to which radio resources are allocated in a certain order such that new IDs are allocated to the UEs in accordance with the transmission sequence numbers of the UE IDs. As the new IDs are mapped to the allocated resource blocks in a one-to-one correspondence, the UEs can find out which resource blocks are allocated to them. Therefore, signaling overhead is decreased, compared to the conventional signaling of information about allocated resource blocks together with the full UE IDs to the UEs.
In the illustrated case of FIG. 4, new IDs 01, 10 and 11 are allocated respectively to first, second and third UEs (301, 302 and 303) (UE 1, UE 2 and UE 3 respectively). If a resource block is not allocated to any UE, 00 is mapped to the resource block. Hereinafter, the new IDs are called as short IDs.
The BS allocates the resource blocks to UE 1, UE 2 and UE 3. A resource block 307 is allocated to UE 2 and thus 10 is signaled in respect to the resource block 307. Since the following resource block 308 is not allocated to any UE, 00 representing none is signaled in respect to the resource block 308. A resource block 309 is allocated to UE 1 and thus 01 is signaled in respect to the resource block 309. A resource block 310 is not allocated to any UE, 00 is signaled in respect to the resource block 310. A resource block 311 is allocated to UE 3 and thus 11 is signaled in respect to the resource block 311. For a resource block 313 allocated to UE 2, 10 is signaled.
Therefore, the BS signals the UEs' short IDs for the respective resource blocks, as denoted by reference numeral 306. The signaling of the short IDs in the order of the UEs to which the resource blocks are allocated obviates the need for additional information for mapping between the resource blocks and the UEs because the UEs are aware that a first resource block is allocated to the UE with a first short ID and the other resources blocks are allocated in this manner.
For the above-described resource assignment information signaling, a control channel is configured as described in Table 1 below.
TABLE 1FieldBitsNum of scheduled UEs 5 {Num of scheduled UEs occurrences UE ID10 } {number of resource block Short ID┌log2 (NumofscheduledUEs + 1)┐ }
In Table 1, Num of scheduled UEs indicates the number of UEs to which resources are allocated by the control channel. UE IDs for as many of these fields are included. UE ID identifies a UE to which resources are allocated. As many Short IDs as the allocated resource blocks are signaled. Since the UEs to which the resources are allocated are numbered with the Short IDs, the Short IDs may vary depending on UEs to which resources are allocated by the control channel.
For instance, if three UEs are allocated resources by the control channel, they are represented by 2-bit short IDs. For five UEs to which resources are allocated, 3-bit short IDs are feasible. Notably, more short IDs than the number of the UEs to which resources are allocated are required in order to notify non-allocated resource blocks. If the notification is not made, the number of short IDs can be equal to that of the UEs to which resources are allocated.
While the conventional signaling method reduces signaling overhead in that there is no need for signaling both UE IDs and resource block information, if the sequence of short IDs are not mapped to allocated resource blocks in a one-to-one correspondence, the receivers cannot determine a resource allocation sequence automatically. Moreover, the none ID (00 in FIG. 4) should be included in the short ID sequence to indicate a non-allocated resource block.
Meanwhile, signaling of dedicated resource assignment information that is kept valid for a predetermined time to UEs is under active discussion for services such as Voice over Internet Protocol (VoIP). In this case, there is no need for sending resource block information to the UEs at each time because in uplink transmission, a scheduler and a transmitter reside in different nodes.
Synchronous Hybrid Automatic Repeat reQuest (HARQ) uses the same resources and the same Adaptive Modulation and Coding (AMC) level for a retransmission as in an initial transmission. In this case, there is also no need for sending resource assignment information on a control channel. Since no IDs are sent even when the resource assignment information is not required, the conventional control channel transmission method is inefficient.
When control information is delivered to UEs on a plurality of Shared Control Channels (SCCHs), the conventional one-to-one mapping between short IDs and resource blocks is notified by each of the SCCHs, thereby causing signaling overhead. Accordingly, there exists a need for efficiently signaling resource assignment information, i.e. control information.